Alkyl tin compounds are widely used as a variety of organic synthesis catalysts. The subset of dialkyl tin dialkoxides are highly useful as catalysts such as ester synthesis catalysts, carbonic acid ester synthesis catalysts, transesterification reaction catalysts and silicone polymer or urethane curing catalysts. Carbonic acid esters are used as additives, such as gasoline additives for the purpose of increasing octane value, and diesel fuel additives to reduce particles in exhaust gas, while they are also useful compounds as alkylating agents, carbonylating agents, solvents and the like for synthesis of organic compounds such as polycarbonates and urethanes, drugs and agricultural chemicals, or as lithium battery electrolytes, lubricant oil starting materials or starting materials for deoxidizers for rust prevention of boiler pipes, and the aforementioned dialkyl tin dialkoxides are of particular interest as catalysts for their synthesis.
In PTL 1 there is disclosed a method for producing carbonic acid esters by thermal decomposition of an addition product formed by reacting an organometallic compound comprising a dialkyl tin dialkoxide with carbon dioxide.
In the prior art there have been known methods for producing dialkyl tin dialkoxides by dehydrating reaction of dialkyl tin oxides and alcohols, and removal of the low boiling point components including the generated water from the reaction mixture (see PTLs 2 to 4 and NPLs 1 to 2, for example). Methods for producing dialkyl tin dialkoxides by dehydrating reaction between dialkyl tin oxides and alcohols are assumed to be equilibrium reactions occurring with dehydration, represented by chemical equation (5).
[In the equation, R and R′ represent alkyl groups.]
The equilibrium reaction is overwhelmingly unbalanced toward the left, and is presumably often accompanied by successive dehydrating reaction via a tetraalkyldialkoxydistannoxane, as represented by chemical equations (6) and (7).
[In the equation, R and R′ represent alkyl groups.]
[In the equation, R and R′ represent alkyl groups.]
In order to obtain a dialkyl tin dialkoxide at high yield, production is carried out while removing the water generated by the dehydrating reaction out of the system, but since this reaction is disadvantageous in terms of energy, the reaction must be carried out for a long period at high temperature (for example, 180° C.). Even when dialkyl tin dialkoxides are used as catalysts for other esterification reactions and urethanation reactions, they are often used at high temperatures exceeding 100° C.
On the other hand, it is known that heating dialkyl tin alkoxide compounds (such as dialkyl tin dialkoxides) to about 180° C., for example, generate modified forms such as trialkyl tin alkoxides having three alkyl groups on one tin atom (see NPL 2, for example). While it is not clear by what reaction the trialkyl tin alkoxides are generated, it is believed that alkyl groups are transferred within the molecule. For example, when the alkyl tin alkoxide is a tetraalkyldialkoxydistannoxane, formation of modified compounds (trialkyl tin alkoxides and high boiling point tin components) by the disproportionation reaction represented by chemical equation (8) has been confirmed, and when the dialkyl tin alkoxide is a dialkyl tin dialkoxide, formation of modified compounds (trialkyl tin alkoxides and high boiling point tin components) by the disproportionation reaction represented by chemical equation (9) has been confirmed, while production of trialkyl tin alkoxides has also been confirmed with the passage of time during synthesis of dialkyl tin dialkoxides from dialkyl tin oxides and alcohols and during synthesis of dialkyl tin dialkoxides from tetraalkyldialkoxydistannoxanes and alcohols. Throughout the present specification, “tin component” refers to a compound containing all of the tin atoms contained in a reaction mixture or composition.

From Chemical Equation (8) it is inferred that a trialkyl tin alkoxide and a monoalkyl tin compound having one alkyl group on one tin atom are generated as modified forms of a tetraalkyldialkoxydistannoxane. The present inventors have in fact confirmed that trialkyl tin alkoxides and high-boiling-point tin components are present among the modified forms of tetraalkyldialkoxydistannoxanes, and it is conjectured that the high-boiling-point tin components correspond to monoalkyl tin compounds.
However, the structures of the high-boiling-point tin components presumed to correspond to monoalkyl tin compounds have not yet been elucidated. Modified compounds presumed to be trialkyl tin alkoxides and monoalkyl tin alkoxides are produced from dialkyl tin dialkoxides as well, but the structures of the modified compounds presumed to be monoalkyl tin alkoxides have not yet been elucidated.
Generation of such modified compounds has also been confirmed in the course of production of the dialkyl tin dialkoxides mentioned above, and in the course of production of carbonic acid esters by thermal decomposition of addition products formed by reacting carbon dioxide with organometallic compounds containing dialkyl tin dialkoxides.
Trialkyl tin alkoxides are known to have very low ability to produce carbonic acid esters, in production of carbonic acid esters by reaction of carbon dioxide with tin compounds (see NPL 3, for example). High-boiling-point tin components, which are present among the aforementioned modified compounds and whose structures cannot be identified, also have very low ability to produce carbonic acid esters in production of carbonic acid esters by reaction between carbon dioxide and tin compounds (see PTL 4, for example).
Thus, since modified compounds do not exhibit high reactivity in production of carbonic acid esters by reaction between carbon dioxide and tin compounds, when such modified compounds are generated during production of such carbonic acid esters, repeated use of the alkyl tin alkoxide compounds will result in accumulation of modified forms of dialkyl tin alkoxide compounds with low activity and decrease in the proportion of active dialkyl tin alkoxide compounds, often lowering the reaction rate and reducing carbonic acid ester yields. In such cases, small amounts of fresh dialkyl tin alkoxide compound are added in order to maintain a constant reaction rate or yield for most reactions, but if fresh dialkyl tin alkoxide compounds are simply added and the modified compounds are left to remain, this results in the problem of accumulation of large amounts of degradation products with low activity (modified compounds) in the reaction system. Even when some mixtures of alkyl tin alkoxide compounds containing modified products of dialkyl tin alkoxide compounds are extracted from the reaction system while fresh dialkyl tin alkoxide compounds are added to maintain a constant concentration of dialkyl tin alkoxide compounds in the reaction system, the modified dialkyl tin alkoxide compounds that have been extracted not only constitute waste, but active dialkyl tin alkoxide compounds are also extracted at the same time as waste, and this results in major problems in terms of cost and disposal.
Several solutions have been presented to counter this problem (see PTLs 5 to 6, for example). Specifically, PTL 5 discloses a method in which, during production of carbonic acid esters using dialkyl tin alkoxide compounds containing modified forms of dialkyl tin alkoxide compounds, the trialkyl tin compound component is separated by distillation from the dialkyl tin alkoxide compounds that include the modified compounds, thereby preventing their accumulation in the reaction system. However, high-boiling-point tin components whose structures cannot be identified, and which are present among the modified forms of dialkyl tin alkoxide compounds, cannot be removed from the reaction system and therefore it is not possible to completely prevent accumulation of modified dialkyl tin alkoxide compounds by this method.
The present inventors have previously disclosed a method for separating and recovering products derived from dialkyl tin alkoxide compounds as dialkyl tin dialkoxides, by reacting alcohols and/or carbonic acid esters with the mixtures of dialkyl tin alkoxide compounds and modified forms of dialkyl tin alkoxide compounds that have been extracted from the reaction system (see PTL 6). This method solved the problem in which the active dialkyl tin alkoxide compounds are disposed with the modified compounds, allowing selective disposal of only the modified forms of the dialkyl tin alkoxide compounds. However, since modified dialkyl tin alkoxide compounds cannot be reused, the problems of cost and waste have remained.
The present inventors have further disclosed a method of highly efficient regeneration of dialkyl tin dialkoxides, by subjecting tin compounds obtained from disproportionation reaction, to redistribution reaction (see PTL 7). We have further disclosed a method of producing carbonic acid esters over long periods without loss of productivity, by incorporating the aforementioned redistribution reaction step as a step in carbonic acid ester synthesis processes (see PTL 8).
However, it has not been possible to prevent the inactivation of the dialkyl tin dialkoxide compounds themselves, and there is a need for development of dialkyl tin alkoxide compounds that are resistant to disproportionation reaction when used for ester synthesis, urethanation reaction and the like and alkyl tin catalysts with high carbonic acid ester productivity, with minimal disproportionation inactivation particularly during production of carbonic acid esters that have high practical industrial value, and a solution yet remains to be found.